Enzyme action and activity — Edexcel GCSE Combined Science
How enzymes work, why they are denatured, the effect of temperature, substrate concentration and pH, and calculating rates.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Enzymes are proteins that act as biological catalysts, speeding up reactions without being used up. Each enzyme has an active site with a particular shape. In the lock and key model, only a substrate with a shape complementary to the active site can fit, forming an enzyme-substrate complex. This is why each enzyme is specific to one type of reaction.
An enzyme is denatured when a high temperature, or a pH too far from its optimum, breaks the bonds that hold it in shape. The active site changes shape, so the substrate no longer fits and the reaction cannot be catalysed. Denaturing is permanent. Enzymes are not 'killed' by heat, because they were never alive.
Enzyme activity increases with temperature up to an optimum, around 37 °C for many human enzymes, because enzyme and substrate collide more often. Above the optimum the enzyme is denatured and the rate falls. Each enzyme also has an optimum pH. Increasing substrate concentration raises the rate until all the active sites are occupied, then the rate levels off.
This core practical investigates how pH affects the activity of amylase, which breaks down starch. Starch, buffer and amylase are mixed in a water bath, and a drop is added to iodine solution every 30 seconds. When the iodine stays orange-brown, the starch has gone. The shortest time shows the optimum pH; temperature, volumes and concentrations are kept the same.
The rate of reaction is how fast a reaction happens, calculated as the amount of product formed divided by the time taken. Its units depend on what is measured, such as cm³/s for a volume of gas. If the time for a reaction to finish is measured, the rate is 1 ÷ time, in s⁻¹.
Exam questions
7 exam-style questions (18 marks), each with its mark scheme.
Answer the questionsFlashcards
11 cards: flip them, mark what you knew, and practise the rest.
Practise the cardsMind map
The whole of key concepts in biology on one page, so you can see where this subtopic fits.
Open the mind mapPractical sheet
Method, variables, risks and a results table, with questions to answer.
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Quick check questions
What is the name of the part of an enzyme that the substrate fits into?
Show answer
The active site.
Why does an enzyme stop working at a high temperature?
Show answer
The shape of the active site changes, so the substrate no longer fits. The enzyme is denatured.
Why does the rate level off as substrate concentration increases?
Show answer
All the active sites are occupied.
In the pH core practical, what colour shows that starch is still present with iodine solution?
Show answer
Blue-black.
Calculate the rate when 30 cm³ of gas is collected in 60 s.
Show answer
0.5 cm³/s.
Frequently asked questions
How do enzymes speed up reactions?
Enzymes speed up reactions by acting as biological catalysts, without being used up. The substrate collides with the enzyme's active site and fits into it because their shapes are complementary, forming an enzyme-substrate complex. The reaction happens at the active site, the products leave, and the enzyme can be used again.
Why do enzymes denature at high temperatures?
Enzymes denature at high temperatures because the bonds that hold the enzyme in shape are broken. The enzyme and its active site change shape, so the substrate no longer fits and the reaction cannot be catalysed. This change is permanent, so above the optimum temperature the rate falls quickly towards zero.
What happens to enzymes at low temperatures?
At low temperatures enzymes work slowly, but they are not denatured. The particles have less kinetic energy and move more slowly, so the enzyme and substrate collide less often and the rate of reaction is lower. As the temperature rises towards the optimum, collisions become more frequent and the rate increases.
Why are enzymes specific?
Enzymes are specific because each enzyme's active site has a particular shape. Only a substrate with a shape complementary to the active site can fit into it and form an enzyme-substrate complex. So each enzyme catalyses only one type of reaction, which is why the body needs many different enzymes.
What happens to enzymes at high pH?
If the pH is higher than an enzyme's optimum, the rate of reaction decreases. At extreme pH values the active site changes shape and the enzyme is denatured, so the substrate no longer fits. Each enzyme has an optimum pH where it works fastest, and different enzymes have different optimum pH values.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026